[As the Roman Empire demonstrated, a Praetorian Guard is a fine thing, until its leadership starts getting uppity and decides it wants a role in choosing the Emperor. ]
All kings have traditionally had to balance the powers of their military commanders and deliberately set them at each other’s throats for precisely this reason, to keep them too busy to plot against him individually or collectively. Alexander’s Companions whacked his entire family after he died. We get the cleaned up Cliff’s Notes versions of many of these things, but the history was, in reality, wheels within wheels within wheels.
Alexander’s Companions whacked his entire family after he died. Herod the Great whacked some of his own sons, because he feared they were plotting against him. Many kings were like that.
Game of Thrones was inspired by history.